1. Field of the Invention
This invention relates to an anti-reflection coating for optical components and a method of forming the same, and is characterized by forming an anti-reflection coating on an optical component at room temperature by vacuum evaporation without heating.
2. Description of the Prior Art
Heretofore, in case of forming an anti-reflection coating on an optical component, a method has usually been produced wherein an evaporating material for the formation of the anti-reflection coating is deposited on the optical component by heating at 250.degree.-400.degree. C. under vacuum, or the so-called hot coat method. In the hot coat method, however, the application range of vacuum evaporation, productivity, and stability of properties are very narrow and low at present as will be mentioned later.
For instance, when an inorganic glass substrate is used as the optical component, there are the following drawbacks (1)-(6):
(1) Since a metal fluoride, metal oxide or the like is used as the evaporation material, an undesirable oxidation reaction is caused on the heated glass substrate by the hot coat method, or the resulting deposit coating is reoxidized when being exposed to the atmosphere at an incompletely cooled state. As a result, the change of refractive index is revealed as a change of spectral reflectance with the lapse of time. PA1 (2) In case of a high refracting glass substrate containing lead or the like, the metallic element such as Pb or the like is apt to be separated on the substrate surface by heating. As a result, the spectral reflecting characteristics of the Pb separated layer are obtained in addition to the spectral reflecting characteristics of the deposition coating, so that the anticipated spectral reflectance of the coating cannot be obtained. PA1 (3) When a glass having a considerably high linear expansion coefficient such as FK-01 or the like is used as the substrate, it is necessary that the heating temperature during the evaporation be set at a value lower than the usual one or the slow cooling time after the heating is prolonged so as to prevent the cracking of the glass itself. PA1 (4) In vacuum evaporation, the glass substrate is usually arranged on a metal fitting and placed in a dome of an evaporation apparatus. When the hot coat method is carried out at a temperature of about 300.degree. C. and then cooling is performed, a difference in shrinkage between the glass and the metal fitting results in accordance with the difference of linear expansion coefficient therebetween. Consequently, when the size of the clearance between the glass substrate and the metal fitting is very small, the metal fitting invades the glass substrate to cause breaking of glass. PA1 (5) In the vacuum evaporation apparatus used in the hot coat method, the temperature is raised by heating from the time of rough evacuation, so that gas generates in the chamber and a very long evacuation time is required. Further, since a slow cooling is required after the evaporation, the total evacuation time becomes very long. PA1 (6) Considering the structural design on the driving, transferring and rotating of the evaporation apparatus as well as the chamber in itself, troubles such as deformation and the like may be caused by the heating (about 300.degree. C.) in the vacuum seal, lubricating system and bearings. For instance, when the bearing is used in the driving part, it is apt to cause baking and a long-period of durability is not satisfied. Therefore, a cooling portion is arranged in the driving part, or the bearing may be made from a special material or subjected to a special treatment, so that the cost of parts used becomes very expensive. As the vacuum seal (packing) material is used having a high heat resistance, which also results in the increase of the cost. Further, if it is intended to use an apparatus of a complicated mechanism having a transferring system and a rotation system, it is necessary to arrange cooling portions in these systems as a countermeasure for the heating, and consequently the amount of water used in these cooling portions affects the cost. And also, it is necessary to use a special and thick material capable of reducing the releasing gas without causing material deformation, which results in an increase of the cost. PA1 I. SiO.sub.2 and Si.sub.2 N.sub.4 are evaporated in the same amounts, while controlling the ratio of evaporation rates, to form an oxygen silicon nitride film having a desired refractive index; PA1 II. SiO is evaporated by reaction with the atmosphere such as NO, ammonia or the like; PA1 III. An evaporation source such as Si metal or SiO is subjected to reactive ion plating in a gaseous atmosphere such as Ar+NO, ammonia, N.sub.2, O.sub.2 or the like; and PA1 IV. A target of Si metal is subjected to sputtering evaporation in a reactive gas atmosphere such as Ar+NO, ammonia, N.sub.2, O.sub.2 or the like. PA1 (i) A very long time is taken when a hard coat film with a thickness of 1-3.mu. is evaporated by the reactive high-frequency ion plating process; PA1 (ii) It is very difficult to evaporate an oxygen silicon nitride film having an optional refractive index with a good reproducibility by high-frequency ion plating in an apparatus containing an active or inert gas atmosphere such as Ar, NO, ammonia, N.sub.2, O.sub.2 or the like; PA1 (iii) In the formation of an oxygen silicon nitride film by reactive high-frequency ion plating, the evaporation rate cannot be made large, so that the evaporation time is too long. Further, when the distance between the evaporation source and the substrate is sufficient to avoid radiation heat from the evaporation source, the evaporation time is required to be more prolonged. However, then the substrate is exposed to a high-frequency plasma atmosphere; if the evaporation takes a long time, the surface temperature of the substrate always tends to be raised. Since the substrate is not resistant to heat, the accuracy of the shape and size of the substrate is decreased by the long-time required for evaporation and finally the shape of the substrate is deformed. PA1 (iv) In high-frequency ion plating, the region of film formation distribution is narrow, so that mass production is very poor. Further, the apparatus used in itself is mechanically complicated, so that the operation manual is difficult and the cost is high. Particularly, since plasma discharge is utilized, it is difficult to quantify parameters and the reproducibility of the film properties is very poor.
Next, when a synthetic resin substrate is used as an optical component in the hot coat method, drawbacks result as follows.
The transparent synthetic resin for optical use includes acrylic resin based diethylene glycol bisallyl carbonate or polycarbonate (hereinafter abbreviated as PC), acrylic resin, acrylonitrile-styrene copolymer (hereinafter abbreviated as AS), polystyrene (hereinafter abbreviated as PS) and the like. The optical components are cheaply manufactured from these resins by casting, injection molding or the like, so that they are light as compared with the inorganic glass components and can freely be made into an optional shape, but are weak in heat (deformation, deterioration of material properties) and are easily injured. This weak point in heat is a fatal defect in the formation of the anti-reflection coating by the vacuum evaporation of the hot coat method. That is, in case of the inorganic glass substrate, a coating of metal fluoride or metal oxide having high adhesion and hardness can easily be evaporated on the substrate by heating to 250.degree.-400.degree. C. Moreover, in case of the synthetic resin substrate, heating above 80.degree. C. deforms the substrate or changes the refractive index thereof, so that it is very difficult to form an anti-reflection coating having high adhesion and hardness by vacuum evaporation under heating at 250.degree.-400.degree. C.
In order to form the anti-reflection coating on a synthetic resin optical component, therefore, various methods have been attempted, as will be mentioned later, instead of the hot coat method. However, these methods take a serious view of the coating material, coating formation, coating properties (optical characteristics, durability) different from the case of forming an anti-reflection coating on inorganic glass, so that they have various drawbacks as mentioned below.
(a) The first method is a method of depositing SiO.sub.2 or evaporation glass as an undercoat layer on the surface of the synthetic resin optical component at a thickness of 0.5-10.mu. and then forming an anti-reflection coating thereon. In this method, however, the deposition of SiO.sub.2 or evaporation glass takes a long time, so that the workability and productivity are poor. Further, when the thickness of the layer of SiO.sub.2 is 0.5-10.mu., the absorption of light is increased and a 0.1-0.3% ripple reflection waveform is exhibited. And also, since the strength of the undercoat layer is great, stress deformation is caused in accordance with the thickness of the undercoat layer, whereby the accuracy of the shape of the synthetic resin optical component can not be retained satisfactorily.
(b) The second method is a method wherein a polytrialkoxy silane is applied to the surface of the synthetic resin optical component to form a hardened coating or further an inorganic oxide coating of SiO, SiO.sub.2, Al.sub.2 O.sub.3 or the like is formed thereon by vacuum evaporation. In this method, however, it is very difficult to apply the polysiloxane paint to the optical component having an optional shape at an accuracy of uniform thickness. Further, the cost of equipment (production) for forming the strong hardened coating in a short time is expensive.
Up to now, when the above method is applied to a plate-like optical component, the thickness of the hardened coating formed in a short time is limited to about 1.mu. in order to retain the high accuracy of such thickness without changing the accuracy of the component shape. On the other hand, in case of the synthetic resin optical components having an optional shape, it is almost impossible, when the polysiloxane paint is applied to the surface of the optical component at a thickness of 0.1-0.3.mu. as a whole, to obtain a strong hardened coating in a short time without changing the accuracy of the component shape.
(c) The third method is an evaporation of SiO on the synthetic resin optical component. SiO can be evaporated by resistance heating, and has good adhesion to the optical component as compared with SiO.sub.2, and can optionally change the refractive index by the enclosing of O.sub.2, and can impart optical characteristics as a multi-layer coating. However, it is difficult to control the evaporation rate of SiO from the beginning of evaporation to the completion thereof in proportion to the heating force during the evaporation of SiO. Furthermore, an oxidation action of SiO or a difference in the change of refractive index is produced by the size of the chamber, evacuation amount and evacuation rate in the evaporation apparatus during the changing of refractive index through O.sub.2, so that it is difficult to always obtain the desired refractive index with good reproducibility.
Therefore, the evaporation of SiO is very poor in productivity (automation, mass production), which comes to the problem of considering high efficiency and yield.
(d) The fourth method is a method wherein a hard coat film consisting of an oxygen silicon nitride (SiO.sub.x N.sub.y) and having chemical stability and no hygroscopicity is formed on the synthetic resin optical component by a high-frequency ion plating process and then a coating having an anti-reflection effect is formed thereon. In this case, the oxygen silicon nitride film is formed by any one of the following four processes.
In any case of the above processes, an oxygen silicon nitride film having a refractive index of 1.46-2.00 can be formed, so that when this film is adhered as a first layer on the optical component in compliance with the refractive index of the optical component, it can be said to provide a sharp and hardenable anti-reflection coating having no ripple waveform as the spectral reflectance produced by the difference of refractive index with good reproducibility. However, the synthetic resin optical component to be subjected to such a reactive high-frequency ion plating process is usually a thermosetting resin substrate such as CR-39 or the like. This thermosetting resin may be durable at a high temperature of 100.degree. C. as compared with the injection molding resin, so that according to the fourth method, CR-39 or the like practically used in spectacle lens is previously heated to about 70.degree. C. before the beginning of evaporation. Therefore, the following drawbacks result when a hard coat film of oxygen silicon nitride is formed at a thickness of 1-3.mu. and then an anti-reflection coating is formed thereon.